This Invention relates to a self-oscillation circuit for driving an oscillatory actuator, and more particularly to a dual sided self-oscillation circuit for driving a linear or rotational oscillatory actuator which causes drive current in two directions in the actuator within a resonant cycle of the actuator for improving performances and controllability of the actuator.
A linear oscillatory actuator has been utilized in many electrical devices such as a reciprocatory shaver, and normally comprises a stator carrying a winding and a reciprocator carrying a permanent magnet. The winding is connected to receive a periodical electric current from a power source to generate a magnetic flux which interacts with the permanent magnet to cause a mechanical resonance of the reciprocator around its natural frequency, thereby forcing the reciprocator to move linearly relative to the stator. This invention is directed to a self-oscillation circuit for driving a linear oscillatory actuator or other types of actuator such as a rotational oscillatory actuator. Although a reciprocator is part of an actuator, within the context of this invention, the terms xe2x80x9cactuatorxe2x80x9d and xe2x80x9creciprocatorxe2x80x9d may interchangeably be used.
U.S. Pat. No. 6,133,701 discloses a system for driving the linear oscillatory actuator with a self-oscillation circuit, one of circuit diagrams therein is shown in FIG. 1. The circuit is connected to receive a back electromotive force voltage signal developed across the winding (actuator coil) in a positive feedback manner to generate a drive pulse. The electric current generated by the drive pulse is periodically supplied to the winding for continuing the mechanical resonance of the reciprocator. With this scheme, however, the reciprocator is difficult to keep the consistent oscillation without being considerably damped when subjected to a heavy load.
Further, in the prior art example shown in FIG. 1, various performances of the circuit such as power control, self start oscillation and etc. are not sufficient for next generation circuit design. The present invention has been accomplished in view of the above background to provide an improved self-oscillation circuit for driving a linear or rotational oscillatory actuator around its resonant frequency.
It is, therefore, an object of the present invention to provide a dual sided self-oscillation circuit for driving an oscillatory actuator which is able to generate two drive pulses to cause electric current flowing in positive and negative directions in the actuator per resonant cycle of the actuator.
It is another object of the present invention to provide a dual sided self-oscillation circuit for driving an oscillatory actuator for improving the performance of the actuator such as reduction of power consumption and increase in response speed.
In the present invention, the self-oscillation circuit for driving an oscillatory actuator which has a winding to receive a periodical supply current from a power source and oscillates in a predetermined resonant frequency, includes a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, and a power amplifier for receiving the sine wave output signal from the bandpass filter and producing two drive pulses in each cycle of the resonant frequency of the actuator to cause the periodical supply current flowing in positive and negative directions through the winding.
In another aspect of the present invention, the power amplifier is realized by a comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing the drive pulse when the sine wave exceeds the threshold voltage, and a switch connected in series with the winding to connect or disconnect the power source to the winding in response to the drive pulse, thereby causing the periodic supply current flowing in positive and negative directions through the winding.
In a more particular implementation of the present invention, the self-oscillation circuit is configured with a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a first comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing a first drive pulse when the sine wave exceeds the threshold voltage in a first half cycle of the resonant frequency, a second comparator for comparing a sine wave which is inverted in polarity from the sine wave output signal from the bandpass filter with the threshold voltage and producing a second drive pulse when the sine wave exceeds the threshold voltage in a second half cycle of the resonant frequency, and an H-bridge switch circuit having four switches with the actuator connected in a middle portion thereof and connected to the power source. The H-bridge switch circuit connects or disconnects the power source to the winding in response to the first and second drive pulses, thereby causing the periodic supply current flowing in the positive and negative directions through the winding.
Preferably, the threshold voltage of the first and second comparators is different from a bias voltage of the bandpass filter to produce the drive pulses with duty ratio less than 50xe2x80x9450. The self-oscillation circuit additionally includes means for instantaneously changing the threshold voltage of the first and second comparators to be the same as the bias voltage of the bandpass filter at a start-up process of oscillation of the actuator.
In a further aspect, the self-oscillation circuit is comprised of a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing a drive pulse with a positive voltage swing in a first half cycle of the resonant frequency and with a negative voltage swing in a second half cycle of the resonant frequency produced every time when the sine wave crossing the threshold voltage, and a push-pull switch circuit having two switches with the actuator connected in a middle portion thereof to a ground and connected to positive and negative power sources. The push-pull switch circuit connects or disconnects the positive and negative power sources to the winding in response to the drive pulse, thereby causing the periodic supply current flowing in the positive and negative directions through the winding.
In a further aspect, the self-oscillation circuit includes a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a first comparator for comparing the sine wave output signal from the bandpass filter with a first threshold voltage and producing a first drive pulse when the sine wave exceeds the first threshold voltage in a first half cycle of the resonant frequency, a second comparator for comparing the sine wave output signal from the bandpass filter with a second threshold voltage and producing a second drive pulse when the sine wave exceeds the second threshold voltage in a second half cycle of the resonant frequency, and a push-pull switch circuit having two switches with the actuator connected in a middle portion thereof to a ground and connected to positive and negative power sources. The push-pull switch circuit connects or disconnects the positive and negative power sources to the winding in response to the first and second drive pulses, thereby causing the periodic supply current flowing in the positive and negative directions through the winding.
In the self-oscillation circuit of the present invention, the double sided drive method is used in which the drive pulse occurs two times per cycle of the mechanical resonance frequency for causing electric current in positive and negative directions in the actuator winding. This method requires a substantially smaller amount of electric power for driving the actuator than that required in the conventional technology. Further, the double sided drive method of the present invention introduces other benefits such as quick response to external loads, because the repetition rate of the drive pulse is two times higher than that of the single sided drive method.